The incidence of obesity has been rapidly increasing in the United States and other developed countries. Our hypothesis is that changes are occurring in the indigenous microbial populations that are of ancient origin, due to modernization including antibiotic exposure. Our hypothesis is that these changes in the composition of the microbiome are at least in part playing a role in the increasing incidence of obesity. More specifically, we also hypothesize that common antibiotic treatments are incidentally causing changes in the composition of metabolically active gastrointestinal bacterial populations, and that the effects may be sequential in which maternal changes are then inherited by the next generation. We will test this hypothesis by studying both the colonic microbiota, and Helicobacter pylori, the dominant gastric bacteria that interact with gastric epithelium, which produces hormones (leptin and ghrelin) that are involved in energy homeostasis. We propose 5 studies. Study A1 will be to assess the association of maternal H. pylori status on the development of childhood obesity in a presently enrolled cohort in The Netherlands. Study A2 will examine the relationship in children of H. pylori status and the physiology of gastric hormones, inflammation, and immunity. Study A3 will assess the changes in hormonal and metabolic phenotypes in young adults due to clinically indicated eradication of H. pylori. Study B1 will assess the effects on young mice of the continuous administration of low antibiotic doses in terms of metabolic and hormonal phenotypes. Study B2 will examine the development of the intestinal microbiota over the first year of life in singletons and twins, and will assess the effects of antibiotic perturbation on the developmental process. In total, these studies will examine the relationship changing gastric and colonic bacterial population composition with metabolic phenotypes, and provide opportunities for further exploration.